It is known from, for example, GB-A-2,368,819 to provide a ride down mechanism for a steering column assembly comprising a mounting plate for fastening to part of a body structure of a motor vehicle and a skid bracket fastened to the mounting bracket by a pair of threaded fastening means which are engaged with an elongate slot in the mounting plate. The advantage of such an arrangement is that the force required to produce relative sliding between the skid bracket and the mounting plate is set by the torque applied to the threaded fastening means and, by careful tightening of the fastening means, the force required to produce collapse of the ride down mechanism can be accurately set to a relatively low level so that in the event of an accident the steering column is readily collapsed.
It is however a problem with such an arrangement that abuse loads input by an operator of the motor vehicle, particularly during adjustment of the position of the steering column assembly, may allow the fastening means to loosen thereby reducing the clamping force with adverse effects on both collapse performance and perceived quality due to steering column rattle. It is also a potential problem with such an arrangement that partial collapse of the ride down mechanism can be produced if very high abuse loads are applied during adjustment of the steering column and this could have an adverse effect in the event of a crash due to the reduced ride down travel remaining in the ride down mechanism.
It has therefore been proposed to use a ride down mechanism as shown in FIGS. 2 and 3. The ride down mechanism comprising of a mounting plate 20 having four holes 21 therein to secure it to a cross car beam (not shown) and a carriage member 24 which is clamped to the mounting plate 20 by a threaded fastener (not shown) that is engaged with a longitudinal slot 22 in the mounting plate 20 and an aperture 28 in a skid plate 25 part of the carriage 24.
The carriage 24 also has two side plates 26a, 26b in each of which is formed a hole 27 used to support the shaft of an adjustment lever (not shown). Two capsules 29a, 29b are fastened to the skid plate 25 part of the carriage 24 for engagement with recesses 23a, 23b in a rear edge of the mounting plate 20.
The capsules 29a, 29b prevent the movement of the fastener in the slot 22 and therefore prevent loosening of the fastener due to abuse loads input by an operator of the motor vehicle and also prevent loss of ride down travel due to abuse loading.
In the event of a crash, the force applied to the steering column is sufficient to shear the capsules 29a, 29b off the skid plate 25 thereby allowing the skid plate 25 to slide along the mounting plate 20 against the resistance to motion produced by the clamping force therebetween due to the threaded fastener.
It is however a disadvantage with such an arrangement that a relatively high force has to be input into the capsules in order to shear them off because their fixing must be made sufficiently secure to resist any abuse loads transferred during adjustment of the steering column. In the event of a crash this is undesirable because it produces a skewed resistance in which there is a peak in resistance force at the commencement of impact (due the additional force needed to shear off the capsules) followed by a sudden drop to a lower more acceptable resistance after the capsules have been sheared off. This can result in a high occupant retardation rate during initial impact which is not beneficial to the occupant impacting upon the steering column.
It is an object of this invention to provide a ride down mechanism for a steering column assembly that overcomes at least some of the disadvantages associated with the prior art.
According to the invention there is provided an adjustable steering column assembly for a motor vehicle characterised in that the assembly comprises a lower column member slidingly engaged at one end with an upper column member, a ride down mechanism connecting the upper column member to a fixed part of the motor vehicle so as to permit ride down of the upper column member when a force above a predetermined level is applied to the upper column member, an adjustment mechanism having a locked state in which the upper column member is secured in one of a number of positions and an unlocked state in which the upper column member can be moved by an operator within a predetermined range of movement wherein when the adjustment mechanism is in the unlocked state a load transfer means is engaged to bypass the ride down mechanism so as to prevent the transfer of axial load from the upper column member through the ride down mechanism during adjustment of the upper column member.
The ride down mechanism may comprise a mounting bracket fixed to part of the body structure of the motor vehicle, a carriage connected to the upper column member, a clamping means to apply a predetermined clamping force between the mounting bracket and the carriage so as to inhibit relative sliding between the carriage and the mounting bracket during normal use but allowing relative movement between the carriage and the mounting bracket when an axial force above a predetermined magnitude is applied to the upper column member in the event of a vehicle crash wherein the clamping means is a fastener passing through an aperture in one of the carriage and the mounting bracket and a longitudinal slot in the other of the carriage and the mounting bracket so as to provide a slideable connection therebetween.
Preferably, the fastener may be a threaded fastener that is tightened to provide a desired clamping force.
The carriage may be connected to the upper column member by two rack and pinion sets the racks of which are fastened to the upper column member and the pinions of which are mounted on the shaft the pinions being engageable with the racks to secure the upper column member in one of its adjustment positions.
The load transfer means may comprise a detent that is engaged with the lower column member when the adjustment mechanism is in the unlocked state and which is retracted when the adjustment mechanism is in the locked state.
The detent may engage with one of an aperture in the lower column member and a recess in the lower column member.
The steering column assembly may be a telescopically adjustable steering column assembly and the aperture is a slot of sufficient length to permit the upper column member to move throughout a normal range of travel but acting so as to transfer load directly from the upper column to the lower column member when the upper column member reaches either end of its normal range of travel.
The steering column assembly may be a telescopically adjustable steering column assembly and the recess may be an elongate recess of sufficient length to permit the upper column member to move throughout a normal range of travel but acting so as to transfer load directly from the upper column to the lower column member when the upper column member reaches either end of its normal range of travel.
Alternatively, the load transfer means may comprise a detent that interacts with the mounting bracket when the adjustment mechanism is in the unlocked state so as to prevent relative movement between the carriage and the mounting bracket and is retracted when the adjustment mechanism is in the locked state so as to permit relative movement between the carriage and the mounting bracket when an axial force above the predetermined magnitude is applied to the upper column member in the event of a vehicle crash.
The detent may interact with the mounting bracket by engaging with one of a flange formed as part of the mounting bracket and an aperture in the mounting bracket.
The adjustment mechanism may include an operator operable lever connected to a shaft rotatably supported by the carriage and the detent may be driveably connected to the shaft.
The adjustment mechanism may include an operator operable lever connected to a shaft rotatably supported by the carriage and an abutment surface fastened to the shaft for rotation therewith and the load transfer means may comprise a detent in the form of an end stop wherein, when the operator operable lever is in a position corresponding to the unlocked state the end stop is co-operable with the abutment surface so as to transfer load directly from the upper column member to the end stop and, when the operator operable lever is in a position corresponding to the locked state, the end stop is non-engageable with the abutment surface so as to permit normal ride down to occur.
The abutment surface may be on a flange formed as part of the operator operable lever.
The end stop may be fastened to a bulkhead support used to support a lower end of the lower column member.
As yet a further alternative, the load transfer means may comprise a retention strap having one end connected to the mounting plate and at least one detent formed at an opposite end that interacts with the carriage when the adjustment mechanism is in the unlocked state so as to prevent relative movement between the carriage and the mounting bracket, the at least one detent being retracted when the adjustment mechanism is in the locked state so as to permit relative movement between the carriage and the mounting bracket when an axial force above the predetermined magnitude is applied to the upper column member in the event of a vehicle crash.
Preferably, the strap may have two detents for engagement with the carriage.
The strap may have two detents for interaction with the carriage and the two detents may interact with the carriage by one of spring biasing into contact therewith or positive clamping to the carriage.
The or each detent may be arranged to fail when a force greater than a predetermined force is applied to it so as to facilitate normal functioning of the ride down mechanism in the event of a vehicle crash with the adjustment mechanism in the unlocked state.